1. Field of the Invention
This invention relates to liquid crystal light valves, and more particularly to light valves which are based upon a depletion of majority carriers in a photoconductor layer, and methods of operating the same.
2. Description of the Related Art
Light valves, generally employing liquid crystals as an electro-optic medium, are used to spatially modulate a readout beam in accordance with an input signal pattern applied to the light valve. They can be used to greatly. amplify the input pattern by controlling a readout beam of much greater intensity, to convert spatially modulated incoherent input radiation to a coherent readout laser beam with a similar spatial modulation, for optical data processing, wavelength conversion, or for other purposes that involve the conversion of an input signal pattern to a corresponding spatial modulation on a separate readout beam.
A simplified block diagram of a typical light valve system is illustrated in FIG. 1. An input beam 2 is developed from a source such as the screen of a cathode ray tube 4 and imaged through lens 6 onto the input side of a light valve 8. On the other side of the light valve a readout beam 10 is generated by a laser 12, and directed onto the readout side of the light valve by a polarizing beam splitter 14. The input beam 2 establishes a spatial polarization of a liquid crystal layer within the light valve 8, and this layer controls the reflection of the readout beam from the light valve. Certain portions of the readout beam are incident upon locations in the liquid crystal layer where the liquid crystal molecules have been rotated in response to the voltage generated by the input radiation, and these portions are retro-reflected back through beam splitter 14 to emerge as an output beam 16. In this example, the liquid crystal in the light valve modulates the spatial intensity of the readout beam into a corresponding but amplified intensity pattern of the input beam.
The main parameters of light valves are the input sensitivity, output, and resolution modulation (contrast ratio), as well as output uniformity and frame rate. While high contrast, moderate brightness and color capability are required for command and control displays, very high brightness and resolution, as well as fast response, are required for flight-simulation applications. Optical data processing applications require low wavefront distortion (output uniformity) and high diffraction efficiency. In addition, for real-time portable scene correlators, high frame rate, wide spectral range, small size, and low power consumption are also required. Most of these requirements are met by a cadmium sulfide liquid crystal light valve developed by Hughes Aircraft Company. This device is described in articles by J. Grinberg, A. Jacobson, W. P. Bleha, L. Miller, L. Fraas, D. Boswell and G. Myer, "A New Real-Time Non-Coherent to Coherent Light Image Converter--The Hybrid Field Effect Liquid Crystal Light Valve", Optical Engineering 14, 217 (1975), and J. Grinberg, W. P. Bleha, A. Jacobson, A. M. Lackner, G. Myer, L. Miller, J. Margerum, L. Fraas and D. Boswell, "Photoactivated Birefringent Light-Crystal Light Valve for Color Symbology Display", IEEE Transactions Electronic Devices ED-22, 775 (1975).
The main drawback of the CdS-based light valve has been its slow response time. A second generation silicon-based liquid crystal light valve has been developed which retains the advantages of the CdS-based light valve and has a considerably faster response time. The silicon-based device employs a metal-oxide-semiconductor (MOS) structure, and is described in an article by U. Efron, J. Grinberg, P. O. Braatz, M. J. Little, P. G. Reif and R. N. Schwartz, "The Silicon-Liquid Crystal Light Valve", Journal of Applied Physics 57(4) 1356-68 (1985). This article also summarizes some of the prior light valve efforts.
The internal construction of an MOS light valve is shown in FIG. 2. An input image beam on the right hand side of the device is identified by reference numeral 18, while a readout beam 20 is directed onto, and reflected from, the left hand side of the device. A layer of high resistivity silicon photoconductor 22 has a thin p.sup.+ back contact layer 24 formed on its readout side. This back contact provides a high sheet conductivity to present a very small load at any point in the device's cross-section where carriers are generated. An SiO.sub.2 oxide layer 26 is provided on the input side of back contact 24, with a fiber optic plate 28 adhered to the oxide layer by means of an optical cement 30. A DC-biased n-type diode guard ring 32 is implanted at the opposite edge of the silicon photoconductor wafer 22 from back contact 24 to prevent peripheral minority carrier injection into the active region of the device. An SiO.sub.2 gate insulator layer 34 is formed on the readout side of the silicon photoconductor wafer 22. Isolated potential wells are created at the Si/SiO.sub.2 interface by means of an n-type microdiode array 36. This prevents the lateral spread of signal electrons residing at the interface.
A unified thin film dielectric mirror 38 is located on the readout side of the oxide layer 34 to provide broad-band reflectivity, as well as optical isolation to block the high intensity readout beam from the photoconductor. A thin film of fast response liquid crystal 40 is employed as the light modulating electro-optic layer on the readout side of mirror 38. A front glass plate 42 is coated with an indium tin oxide (ITO) counter-electrode 44 adjacent the liquid crystal. The front of glass plate 42 is coated with an anti-reflection coating 46, and the whole structure is assembled within an airtight anodized aluminum holder.
Silicon photoconductor 22 is coupled with oxide layer 34 and transparent metallic electrode coating 44 to form an MOS structure. The combination of the insulating liquid crystal, oxide and mirror act as the insulating gate of the MOS structure.
In operation, an alternating voltage source 48 is connected on one side to back.. contact 24 by means of an aluminum back contact pad 49, and on its opposite side to counter-electrode 44. The voltage across the two electrodes causes the MOS structure to operate in alternate. depletion (active) and accumulation (inactive) phases. In the depletion phase, the high resistivity silicon photoconductive layer 22 is depleted and electron-hole pairs generated by input light beam 18 are swept by the electric field in the photoconductor, thereby producing a signal current that activates the liquid crystal. The electric field existing in the depletion region acts to sweep the signal charges from the input side to the readout side, and thus preserve the spatial resolution of the input image. The polarized readout beam 20 enters the readout side of the light valve through glass layer 42, passes through the liquid crystal layer, and is retro-reflected by dielectric mirror 38 back through the liquid crystal. Since the conductivity of each pixel in photoconductive layer 22 varies with the intensity of input beam 18 at that pixel, a voltage divider effect results which varies the voltage across the corresponding pixel of the liquid crystal in accordance with the spatial intensity of the input light. As is well known, the liquid crystals at any location will orient themselves in accordance with the impressed voltage, and the liquid crystal orientation relative to the readout light polarization at any particular location will determine the amount of readout light that will be reflected back off the light valve at that location. Thus, the spatial intensity pattern of the input light is transferred to a spatial liquid crystal orientation pattern in the liquid crystal layer, which in turn controls the spatial reflectivity of the light valve to the readout beam.
Active light valve operation takes place only during the depletion phases. It is necessary to reverse the polarity of the applied voltage and thereby intersperse shorter accumulation periods between the depletion periods to prevent any appreciable DC current through the liquid crystal. This is because the liquid crystal tends to decompose under a DC current.
Since the photoconductor layer 22 is photosensitive, a dielectric mirror/light blocking layer 38 is required that will prevent the high intensity readout light from generating spatially unresolved carriers in the photoconductor that would otherwise swamp the signal charge. Typically, the dielectric mirror/light blocking layer 38 must attenuate the readout beam by a factor of about 10.sup.6 or larger, so that the number of carriers accumulated during the active phase due to light leakage through the dielectric mirror/light blocking layer does not approach or exceed the signal charge. It is quite difficult to fabricate a dielectric mirror with this capability. Although an attenuation of 10.sup.7 has been achieved, some applications require greater attenuations, for which adequate dielectric mirrors are not presently available.
As a possible substitute for a dielectric mirror, a recently developed metal matrix mirror has been demonstrated to provide excellent electrical and optical properties for valves operating in the infrared region. This type of mirror is described in the co-pending U.S. patent application Ser. No. 759,004, "Reflective Matrix Mirror Visible to Infrared Convertor Light Valve" by P. O. Braatz, and assigned to Hughes Aircraft Company.
A metal matrix mirror is illustrated in FIG. 3. A matrix of reflective islands 52 is formed on an insulative layer 54 such as SiO.sub.2. The islands 52 are separated from each other by insulating channels so as to avoid shortcircuits across the face of the mirror. The dimensions of the individual islands 52 are determined from a minimum size for adequate reflection, on the order of 5-20 microns, and the resolution or pixel element size for which the light valve is designed. The thickness of the islands depends upon the specific reflective material employed. There is a basic requirement that the free electron density of the reflective material be sufficient to interact with the readout radiation and scatter it back out of the material. Metals such as aluminum or silver or metal/semiconductor compounds such as platinum-silicide may be used.
The MOS light valve described above has several limitations. While the photoconductor is initially deeply depleted, the depletion region gradually collapses (over the order of tens of milliseconds) because of thermal generation effects which deplete the majority carriers. Eventually the voltage drop shifts to the oxide from the photoconductor. Also, the process for applying the SiO.sub.2 layer requires high temperatures in the order of 1000.degree. C. At these temperatures it is difficult to keep the light valve substrate perfectly flat. Any curvature or waviness in the substrate will distort the readout from the valve. Another disadvantage is that a certain amount of sheet conductivity has been noted at the Si/SiO.sub.2 interface. This effect degrades both the resolution and the dynamic range of the device. Furthermore, while a metal matrix mirror is preferable to a dielectric mirror because of its lower impedance, its use has been limited principally to the infrared region. In the visible region the readout light leaks through the channels between the metal islands, causing activation at the underlying photoconductor.
Another type of light valve which is at least potentially capable of even better performance than the MOS light valve is referred to as the double Schottky diode light valve. It is disclosed in a co-pending patent application entitled "Double-Schottky Diode Liquid Crystal Light Valve" by Paul O. Braatz and Uzi Efron, two of the present inventors. The application was filed on July 25, 1985 under Ser. No. 758,917, and is assigned to Hughes Aircraft Company, the assignee of the present invention. This device is illustrated in FIG. 4. It consists of a photoconductor substrate 58 with Schottky diodes formed on either side. On the readout side the metal pads 60 of a metal matrix mirror form a pattern of Schottky contacts with the photoconductor, while on the input side a metal electrode 62 contacts the phase of the photoconductor to form another Schottky diode. A face plate 64 is attached to the input side of electrode 62 by an optical cement 66.
The liquid crystal layer 68, counter-electrode 70 and glass counter-electrode substrate 72 are similar to the MOS device described above. Alignment layers 74 and 76 are provided on either side of the liquid crystal, which is confined by spacers 78.
In contrast to the operation of the MOS device with relatively long depletion and relatively short accumulation periods, the double-Schottky diode light valve is operated with a balanced AC voltage drive 80 applied across the back electrode 62 and counter-electrode 70. In operation, one or the other of the Schottky diodes will be reverse biased at substantially all times, depending on the phase of the voltage source 80 at any given time. This causes the photoconductor 58 to maintain a state of substantially continuous depletion. Thus, the device avoids the inactive accumulation periods necessary with the MOS light valve, and inherently balances the net current through the liquid crystal to zero. It can be fabricated at a much lower temperature than the MOS device, and does not exhibit the sheet conductivity at the photoconductor surface that degrades the MOS operation. However, the metallic back contact 62 has been very difficult to fabricate and has prevented the full realization of the double-Schottky device's potential.